A long-known limitation to performance of high-speed, long-haul optical fiber transmission systems is polarization dependent loss (PDL). Generally, PDL is the power loss of an optical signal propagating over an optical transmission system that is dependent on the polarization state of the optical signal. Typically, PDL is defined numerically as the difference between the maximum and minimum loss over all possible polarization states of the transmitted optical signal.
Typically, PDL is introduced into an optical transmission system by way of the individual components constituting the system, such as optical fibers, amplifiers, couplers, isolators, multiplexers, and the like. While each of these components exhibits a definable PDL, summing the PDL of each component in an optical transmission system typically does not yield an accurate measure of the PDL for the entire system since many different relative orientations between the components are possible. As a result, the PDL of an optical transmission system is measured over the complete assembled system.
Several methods have been implemented or proposed for PDL measurement of a complete optical transmission system, each of which employs a specialized test signal applied at a transmitter of the system and a measurement device at a receiver of the system. Thus, PDL system testing normally involves simultaneous access and control of both ends of the optical transmission system.
Overview
Discussed herein is a method for measuring polarization dependent loss in an optical transmission system. In the method, a first optical signal is generated, and a polarization of the first optical signal is altered over time. The polarized first optical signal is combined with a second optical signal from the optical transmission system to yield a combined optical signal. The combined optical signal is then coherently detected to yield a radio frequency signal, the power of which is measured. The measured power of the radio frequency signal is processed to generate an indication of the polarization dependent loss of the optical transmission system.
Also presented herein is an apparatus for measuring polarization dependent loss in an optical transmission system. The apparatus includes an optical signal generator configured to generate a first optical signal, as well as an optical polarization controller configured to alter a polarization of the first optical signal over time. Also included in the apparatus is an optical coupler configured to combine the polarized first optical signal with a second optical signal from the optical transmission system to yield a combined optical signal. The apparatus also includes a coherent detector configured to generate a radio frequency signal from the combined optical signal, and a power meter configured to measure a power of the radio frequency signal. Also included in the apparatus is a processor configured to process the measured power of the radio frequency signal to generate an indication of the polarization dependent loss of the optical transmission system. A computer-readable medium comprising instructions executable by such a processor for processing the measured power of the radio frequency signal to generate the indication of the polarization dependent loss of the system is also disclosed.